The invention relates to an X-ray examination apparatus for and a method of forming distortion-free X-ray images.
Mobile X-ray examination systems, for example C-arm X-ray systems, are used for given surgical interventions, for example for orthopedic surgery. Because of the mobile components of such an X-ray examination system and the continuously changing exposure circumstances, the X-ray images must be checked regularly for distortions. Methods and devices of this kind are also used for computer-aided surgery.
U.S. Pat. No. 5,772,594 describes a C-arm X-ray system in which a plurality of reference sensors are mounted on the image intensifier, said reference sensors being localized by a position measuring system. The bone to be treated is provided with markers which are localized by said position measuring system. In this X-ray system the image formed is displayed by means of an image processing device and the surgical tools of the surgeon, also being provided with sensors, are localized by the position measuring system and reproduced in the displayed X-ray image of the relevant bone. Changes of the position of the bone which occur during the operation and after the formation of the image are not reproduced.
Images produced by C-arm X-ray examination systems are usually distorted. Such distortions are due on the one hand to the curved surface of the image intensifier and on the other hand to changes of the external magnetic field, for example the terrestrial magnetic field. Moreover, for example, the C-arm is subject to bending due to the weight of the image intensifier and the X-ray source, said bending not being constant. Such bending and distortions lead to changes in the imaging properties and hence to X-ray images containing defects. Mobile C-arms can be moved to a different position or into a different orientation by the surgeon at any time, so that the imaging properties change continuously.
For novel surgical techniques such as CAS (Computer Aided Surgery) it is necessary to know the imaging properties for the C-arm X-ray system completely.
For applications in the field of tomosynthesis and computed tomography it is also necessary to know the exact imaging properties of the relevant X-ray apparatus for the reconstruction of the X-ray images.
Therefore, it is an object of the invention to provide a method and a device that enable correction of the distortions in the patient images.
This object is achieved according to the invention in that the X-ray examination apparatus includes at least one calibration member for forming a reference pattern and a correction unit for correcting distortions in X-ray images on the basis of the pattern of the calibration members actually formed in the patient X-ray image and the reference pattern.
The positions of the housing of the image intensifier and that of the X-ray source relative to a fixed system of space co-ordinates are determined by means of a position measuring device. To this end, reference sensors are mounted on the housings of the image intensifier and the X-ray source, each of said sensors defining a respective image intensifier system of co-ordinates and an X-ray source system of co-ordinates, which systems of coordinates are fixed relative to the relevant housing. Moreover, suitable calibration members are mounted on the image intensifier or on the X-ray source or on both. The position of these calibration members is fixed relative to the respective housing of the image intensifier or of the X-ray source, thus enabling the determination of the reference pattern associated with the calibration bodies relative to the relevant system of co-ordinates. To this end, the position of the calibration members is calculated in the system of co-ordinates formed by the position measuring device.
The calibration members have a fixed, known shape and appearance. They are reproduced and identified as actual patterns, together with the patient, in the patient X-ray image. The imaging properties of the X-ray examination apparatus can be determined on the basis of the known shape and appearance as well as the known position of the calibration members relative to the housings of the image intensifier and of the X-ray source in the calculated reference pattern and the position of the calibration members in the patient X-ray image. The determination of the imaging properties enables correction of distortions which are due, for example, to the curved surface of the image intensifier, to a change of the terrestrial magnetic field, or to bending of the C-arm.
It has also been found that it is also advantageous to measure the position of the calibration members directly by means of the position measuring device. This position is detected directly by means of suitable indicator means, provided with reference sensors, and is applied to the position measuring device by way of the reference sensors. The imaging properties of the X-ray apparatus are then calculated from the known position data of the calibration members and the positions of the calibration members reproduced notably at the edge in the patient X-ray images.
A patient X-ray image wherefrom distortions have been removed can thus be produced by means of a single patient X-ray exposure, without utilizing several calibration exposures or several patient X-ray exposures. The correction of the distortion is performed intra-operatively, so that a distortion-free X-ray image is presented to the observer.
It has been found that in a preferred embodiment of the invention the calibration members are advantageously provided, for example, in a circular pattern in a disc which is transparent to X-rays, said calibration members consisting of X-ray absorbing spheres or wires. The X-ray transparent disc, for example made of Plexiglas, does not absorb the X-rays so that the patient image is not additionally falsified thereby. Because of their high absorption, however, the spheres or wires can be recognized very well in the patient images.
These Plexiglas discs with the circular arrangement of the metal spheres or the crossed metal wires are arranged at a respective defined distance from and in front of the image intensifier and/or the X-ray source. The imaging properties of the X-ray examination apparatus can be determined on the basis of the shifts or differences between the positions of the calibration members in the calculated reference pattern and the pattern of the calibration members actually formed in the patient X-ray image, thus enabling correction of distortions in the patient X-ray image.
In the case of computed tomography (CT) a three-dimensional X-ray image is reconstructed from a plurality of slice images. Because external magnetic fields again act on the CT apparatus and shifts between the X-ray source and the detector have a negative effect on the patient X-ray image, knowledge of the imaging properties of the CT apparatus is necessary so as to reconstruct a distortion-free X-ray image. In this case the calibration members are arranged in such a manner that they are visible in each slice image and that the imaging properties of the CT apparatus can be determined by means of the calculated reference pattern of the calibration members and the pattern of the calibration members actually formed in the patient X-ray image.
In the case of tomosynthesis, also involving the acquisition of slice images, the correction of distortions in accordance with the invention can be performed by utilizing calibration members and determining the imaging properties.